Figure 5. Noise spectra from the borehole seismometer at Site JT-1
(Leg 186) off Sanriku, Japan
(from Suyehiro et al., 1999). The noise level is positioned at a satisfactory level between the High
Noise Model and the Low Noise Model. The rise of noise around 0.01 Hz is known to be
infragravity wave noise induced from a long-period surface wave in the ocean. At Site WP-2, the
borehole seismometer will be installed in the basement so that such serious noise should be
sufficiently suppressed. The seismometer in Hole 1150D (Leg 186) has a vertical (V) and two
horizontal components that are perpendicular to each other. The direction of the horizontal
components could not be determined during installation of the instrument; therefore, H1 and H2
denote the noise spectra from records of two horizontal components of the seismometer.
dB=decibels. m**2 is fortran code for m2 and m**4 is fortran code for m4.